Vibrating ring motor for feeding particulate substances

ABSTRACT

An apparatus for mixing a particulate substance, which may be in chopped or short fibre form, with a matrix metal comprises a container (11) for molten metal (10) having an aperture (12) through which a stream of the metal can fall, a feed motor (20) for feeding the particulate substance into the metal stream, and one or more downwardly inclined nozzles (23) through which jets of atomizing gas are directed on to the combined flow of metal and particulate substance. The feed motor comprising a horizontal ring (20a) which extends about and is coaxial with the molten metal stream, and which has a radial vibration imparted to it so as to form a node on the axis of the ring. The upper surface of the ring may be horizontal or may be inclined downwardly towards the axis and may have annular grooves of ratchet tooth section with the steeper part of the section facing towards the axis. The particulate substance is placed on the upper surface of the ring (20) and is caused to move into engagement with the molten metal stream by the radial vibration of the ring.

BACKGROUND OF THE INVENTION

This invention relates to motors the motor effect of which can beemployed in two different ways giving it an important but by no meansexclusive application in apparatus for manufacturing metal matrixcomposite materials, i.e. metals incorporating particulate substances.The term "particulate substances" is used herein to include substancesin chopped or short fibre form in addition to powdered and granularsubstances.

Very fine powders are difficult to make flow in a controlled manner andone means of producing a flow is to cause the material to vibrate. Thepresent invention is based on the discovery that if a ring disposedsubstantially horizontally is caused to vibrate radially, preferably atits natural frequency, a flowable substance deposited on the uppersurface of the ring moves to the central aperture of the ring andfurther, that material tends during its fall through the ring to movethen towards the axis of the ring under the radial compressing force ofgas present in the hole and vibrating at the same frequency as the ring.

SUMMARY OF THE INVENTION

According to this invention there is provided a motor comprising a ringarranged with its axis in an up and down direction, and having its uppersurface either disposed substantially horizontally or with a downwardinclination towards the axis of the ring, and means for imparting radialvibration to the ring so to form a node on the axis of the ring.

Where the motor is used as a feed motor for a particulate substance,means is provided for directing the substance on to the upper surface ofthe ring.

A hopper for the material may conveniently have its lower end secured tothe ring in a manner to vibrate with the ring.

The upper surface of the ring may advantageously be inclined towards theaxis of the ring and may be smooth or formed with concentric grooves ora spiral groove of saw-tooth section. In one construction the groove orgrooves are of ratchet-tooth section with the steeper face of thesection facing towards the axis of the ring.

A useful way of producing metal matrix composites is to atomize themolten matrix metal by gas or other means and introduce into theatomized spray the second phase which can be in the form of solidmetallic or non-metallic particles or chopped fibres entrained in a gasstream. The combined stream of metal matrix and second phase particlesmay then be directed on to a substrate where it solidifies, or may beallowed to solidify as a powder. This procedure has many advantages butis not free from technical problems. One such problem is to achieve somedegree of penetration of the metal spray with the gas-entrained secondphase to produce a spray in which the second phase is uniformlydistributed so as to give a solidified product which has a uniformlydistributed second phase. A special difficulty arises when the secondphase consists of a very fine powder which may be sub-micron in size. Inthese circumstances it is difficult to add the particles to the metalstream at a uniform controlled rate with the result that the efficiencyof the operation is low.

The invention also provides apparatus for mixing a particulate substancewith a matrix metal comprising, for the addition of the particulatesubstance, a feed motor comprising a ring arranged with its axis in anup and down direction, and having its upper surface either disposedsubstantially horizontally or with a downward inclination towards theaxis of the ring, means for imparting radial vibration to the ring so toform a node on the axis of the ring and means for depositing thesubstance on to the upper surface of the ring, for directing a stream ofthe metal in molten form axially downward through the ring, and meansfor directing one or more downwardly inclined jets of atomizing gas onto the combined flow of molten metal and particulate material.

A process may thus be obtained in which a stream of liquid, which may becoherent or particulate, is passed through a ring, the top surface ofwhich may be sloping inwards and the inner surface of which is notnecessarily parallel, containing in the central aperture of the ring agas which the ring is radially resonating at high frequency, and inwhich a particulate solid is passed on to the top surface of thevibrating ring such that both the particulate solid and the stream ofliquid are forced by the vibration of the ring and the vibration of thegas enclosed in the central aperture towards a central nodal position inthe ring where they are brought together and may or may not intermingle,the combined streams then being atomized either by gas or other means,to form a spray in which the component parts are uniformly distributed.Such a spray may be directed onto a surface where it solidifies to forma spray deposit or may be allowed to solidify in flight to form aparticulate material or may be collected as a liquid containing adispersion of solid particles.

The process is applicable to any metal that can be melted and atomizedinto a stream of liquid particles. Moreover, any powder or chopped fibrecan be used provided the powders or chopped fibre will pass easily intothe central aperture of the ring.

A particular feature of the process is that the powder or chopped fibreis forced by the radial vibration of the ring to move from the hoppertowards the central aperture. This forced movement can be intensified bymachining rings or spirals on the top surface of the resonator.

In an application of the invention to an apparatus to carry out thisprocess the second phase particles and the metal stream are moved to acentral nodal position on passage through a ring that is vibrating in anultrasonic mode. The rapidly vibrating ring has two motor effects.Firstly it causes the particulate solid to move across the top face ofthe vibrating ring towards a central position. The movement can beaccentuated by providing a downward slope on the top face of the ring.Secondly the vibrating ring causes the gas within the ring to vibratecorrespondingly such that a node is formed in the gas at a central axialposition. Any streams of liquid or particulate solid are moved towardsthis node on passage through the ring. In the case of a metal matrixcomposite being made from a powder which may be a ceramic or oxide and astream of liquid metal, the powder is moved rapidly towards the liquidmetal stream across the top face of the ring and is forced into contactwith the liquid metal stream which itself is moved to a centralposition, if not already there, and extended axially. The movement ofboth the powder and the liquid metal into a central nodal position asthey pass through the ring enables the subsequent atomizing to give auniform distribution of the second phase in the spray and ultimately inthe solidified spray deposit.

The ring should vibrate at or close to its natural resonant frequency tomaximise the displacement amplitude. The shape and dimensions of thering should be such as to optimise the radial motion and to avoidfatigue failure but, within this constraint, it is possible to contourboth the external and any internal surfaces to minimise the risk offatigue and the dissipation of vibrational energy. Stainless steel hasbeen found to be a satisfactory material from which to make the ring.

The frequency of vibration is preferably selected to satisfy severalcriteria. Relatively low frequencies such as 50-5000 Hz give rise tounacceptable design constraints and to distressing audible disturbancewhich at high powers can be a serious health hazard. Above 18 kHz thevibration ceases to be audible to most humans. The range of 18-25 kHzusually avoids discomfort or aural damage. Higher frequencies can beused but for a given power input the displacement amplitude iscorrespondingly reduced, the beneficial effect with regard to theinvention is reduced and the power effectiveness is lower. Furthermorethere may again be an unacceptable engineering design constraint in thatthe radially resonant ring may have too small a diameter to be useful.

The invention will now be described in more detail with reference by wayof example to the accompanying diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an axial section a first embodiment of the invention asapplied in an apparatus for mixing a particulate substance with a matrixmetal, and

FIG. 2 is a view similar to FIG. 1 of a modified form of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the matrix molten metal 10, e.g. aluminium alloy,is contained in a tun-dish 11 at the bottom of which is a pouring nozzle12 for the metal. The particulate material, which in this instance maybe silicon carbide in the form of 10 um powder 13, is contained in anannular hopper comprising an inner wall 14 encircling the tun-dish 11,an outer wall 15 and a bottom wall 16. A series of valve apertures 16aare formed in the bottom wall 16, and an annular plate 17 carrying valveelements 18 engaged in the valve apertures can be raised and lowered tocontrol the outlet area of the hopper. The side wall 15 of the hopper issecured to the upper end of a stainless steel ring 20 comprising upperand lower parts 20a, 20b secured together by bolts 21. An annular plenumchamber 22 is formed in the parts 20a, 20b jointly and gas underpressure, which may be a gas to which the metal is inert such asnitrogen or argon, is supplied to the chamber and from the chamber issupplied to a ring of nozzles 23 mounted in the ring 20 and inclineddownward towards the axis of the ring 20. The ring 20 is caused by meansnot shown to vibrate at 20 kHz by way of concentrators and transducersfrom a 3 kw ultrasonic generator, and the ring 20 is so designed that itresonates at the selected vibration frequency.

In operation of the apparatus, the outer wall of the hopper vibrateswith the ring 20 and causes the powder to flow through the valveapertures 16a on to the upper surface of the ring. The radial vibrationof the ring causes the powder to move radially inward along the uppersurface of the ring and into the central aperture at the same time asthe matrix metal 10 flows downwardly through the aperture. A flow of theinert gas above the powder on the upper surface of the ring is caused tovibrate radially when it enters the central aperture of the ring and thepowder is impelled by the vibration into close contact with the streamof metal and in some cases partially to penetrate the metal stream as itaccelerates in a downward direction and attenuates. The combined streamof particles and molten metal is atomized by the jets of gas issuingfrom the nozzles 23 to give a spray having a uniform distribution ofpowder particles. The combined stream may be directed onto a coolsubstrate to form a deposit of a metal matrix composite having a uniformdispersion of the silicon carbide powder in the aluminium alloy.

It will be understood that any metal or alloy can be used as the matrixand any powder or powder mixture or short or chopped fibre can be usedas the added phase.

The resonating ring 20 centralises the metal stream and prevents orreduces sideways break-up. This is a very useful characteristic sincesmall deviations in the metal stream can cause major changes inbehaviour on atomization.

Provision may be made for cooling of the resonating ring 20 by asuitable coolant flowing through passages formed in the ring.

The mechanism of the centralising movement within the central apertureis related to the pattern of vibration of the gas. The molecules of gasin the central aperture are set in vibration and produce a node at acentral position. Any solid or liquid within this aperture is forcedtowards the node, the driving force diminishing as the nodal positionapproached. This causes constriction of a stream of liquid metal and ofany suspended particles. The constriction causes a stream of liquidmetal to become smaller in diameter and elongate usually in a downwarddirection assisted by gravity; the constriction also has the effect ofdriving particles into the attenuating liquid stream. The effect occurswith particles having a very wide range of sizes including sub-micronparticles. This application of the invention is particularly useful withsub-micron particles because their handling and propulsion byconventional means from a hopper towards the liquid metal stream isdifficult if they are not agglomerated into granules.

The velocity of gas within the central aperture of the vibrating ring 20is not important unless the velocity is high. High velocities causeparticles to be propelled so rapidly through the central orifice thatthere is too little time for the centralising forces to operateeffectively. Lower gas velocities, however, may be very useful tomaintain the entrainment of small particles and also to preventblow-back during atomization.

A further important point of note is that the upper surface of the ringneed not be inclined downward towards the axis but may be horizontalsince the motor effect driving particulate material radially inward isstill obtained. Also the ring is not necessarily circular and thecross-section of the ring may be shaped either for concentrating thevibrational effect at the central part or to conform with externalrequirements or for a compromise between the two. In all cases it ishowever necessary, for energy efficient operation, to ensure that theapplied frequency coincides with the current resonant frequency of thering.

In the modified construction shown in FIG. 2, the ring 20 is formed inone piece, and the gas nozzles 23 are formed in a separate hollow ringmember 24 disposed just below and concentrically with the ring. As inthe previous construction the gas jets incline downwardly and towardsthe axis of the ring. This construction has the advantage that the ringis easier to manufacture than the two-part ring and that the ring iseasier to tune to the required frequency and resonates more effectively.It also reduces the likelihood of fatigue failure of the ring but hasthe disadvantage of lengthening the free-fall of the metal and powderbefore atomization takes place.

Also in the arrangement of FIG. 2, the upper surface of the ring isformed with a series of concentric grooves of somewhat saw-tooth form,or preferably of ratchet-tooth form with the steeper face of the toothfacing towards the axis of the ring. A helical groove of similar sectionmay be provided instead of the concentric grooves if desired. Thegrooves drive the powder more effectively towards the central aperturebecause the surfaces vibrate in a horizontal mode. Any verticalcomponent of the vibration has the supplementary effect of causingfluidization of the powder flowing across the upper surface of the ringand promotes uniform distribution of the particles.

In another application of the invention, the feed motor constituted bythe radially vibrating ring may be employed to produce a uniform flow offine powder from a hopper through the central aperture of the ring. Insuch embodiments, the cuter wall of the hopper is secured to the ring ator near the periphery of the ring. Fine powders which do not readilyflow are caused by the vibration of the ring and the hopper to bedeposited on the upper surface of the ring and to flow towards thecentral aperture in a steady stream. When vibration is stopped the flowof powder stops almost instantly. Such an apparatus may operate inconjunction with a weight sensing device to fill containers with apredetermined weight of powder. If the powder has a higher degree offlowability the hopper may be provided with a valve to control the flowof the powder onto the upper surface of the ring.

In another application of the feed motor, the ring is employed inconjunction with a tun-dish 11 of molten metal 10 arranged to flow in astream downward coaxially through the centre of the ring. Provision ismade also for a flow of air or other gas through the centre of the ringabout the flow of molten metal. When the ring is caused to vibrate atresonant frequency the radial vibration of the gas stream about themolten metal within the central aperture of the ring centralizes theflow of molten metal and prevents or reduces sideways break-up of themetal stream. As previously described, this is advantageous where thestream of molten metal is to be atomized to produce either a powder or aspray-formed product.

The radial vibration of the ring is accomplished through a transducersystem in a manner known per se and will not be described here. Theamount of vibrational energy required is an important factor because theradially inward driving force is proportional to the amplitude ofvibrations which is, in most cases, proportional to the energy input, sothat a degree of control of the rate of flow can be obtained byadjusting the power input and hence the amplitude of the vibrations. Ithas been found in practice that in most cases it is necessary to have aninput of at least 1 kW. For dealing with large quantities of materials,energies between 3 and 10 kW may be necessary. The amount of energyrequired also depends on the design of the resonator. A well designedresonator will resonate with a minimum dissipation of energy whereas apoorly designed, or poorly matched one, will be inefficient. Forcontinuous operation it may be necessary to cool the ring to avoid arise in temperature that would change the acoustic properties.

Either external or internal cooling may be used but in the case ofinternal cooling the cooling channels needed either for gas or watercooling must be designed to minimise the deleterious effects on theacoustic performance of the ring.

The equipment need not be used in a completely vertical attitude becausethe centralising effect operates irrespective of gravity and this can bea useful way of deflecting the stream of metal through a small desiredangle. Gravitational effects will, of course, cause deviation of themetal stream and asymmetry of distribution of the particles which is notdesirable in most cases.

We claim:
 1. A feed motor for a particulate substance, comprising:a) aring arranged with a substantially vertical axis and having an uppersurface which is oriented in one of a substantially horizontal and adeclining inward orientation; b) vibrating means for imparting radialvibration to said ring so to form a node on said vertical axis of saidring; and c) directing means for directing the particulate substanceonto said upper surface of said ring.
 2. A motor according to claim 1,wherein:said ring and said means for imparting radial vibration to saidring are adapted to vibrate said ring at its natural resonant frequency.3. A motor according to claim 1, wherein:said ring is stainless steel.4. A motor according to claim 1, wherein:said upper surface of said ringis declined towards said axis of said ring and is formed with one of aseries of coaxial grooves and a spiral groove.
 5. A motor according toclaim 4, wherein:said one of a series of coaxial grooves and a spiralgroove is of saw-tooth section.
 6. A motor according to claim 4,wherein:said one of a series of coaxial grooves and a spiral groove isof ratchet-tooth section with the steeper face of the section facingtowards said axis of said ring.
 7. A motor according to claim 1,wherein:said means for directing the substance on to said upper surfaceof said ring comprises a hopper, said hopper having a lower end securedto said ring in a manner to receive the vibration of said ring.
 8. Amotor according to claim 7, wherein:said lower end of said hopper issecured to an outer edge of said ring.
 9. A motor according to claim 7,further comprising:d) valve means for controlling the flow of materialfrom said hopper onto said upper surface of said ring.
 10. An apparatusfor mixing a particulate substance with a matrix metal, said apparatuscomprising:a) a feed motor for a particulate substance, said feed motorhavingi) a ring arranged with a substantially vertical axis and havingan upper surface which is oriented in one of a substantially horizontaland a declining inward orientation, ii) vibrating means for impartingradial vibration to said ring so to form a node on said vertical axis ofsaid ring, and iii) particulate directing means for directing theparticulate substance on to said upper surface of said ring; b) streamdirecting means for directing a stream of the matrix metal in moltenform axially downward through the ring; and c) jet directing means fordirecting at least one downwardly inclined jet of atomizing gas onto thecombined flow of molten metal and particulate material.
 11. An apparatusaccording to claim 10, wherein:said jet directing means is a ring ofnozzles included in said ring.
 12. An apparatus according to claim 1,wherein:said ring further includes an annular plenum chamber to whichatomizing gas under pressure is supplied and which is in communicationwith said ring of nozzles.
 13. An apparatus according to claim 10,wherein:said jet directing means is an annular member provided with aring of nozzles disposed closely below and coaxially with said ring. 14.An apparatus according to claim 10, wherein:said ring is provided withpassages for a flow of a coolant fluid.
 15. A method of feeding aparticulate substance to a selected location utilizing a ring having acentral axis, an upper surface, and a resonant frequency, said methodcomprising:a) disposing the ring with its axis arranged vertically andin position over the location, and arranging the upper surface of saidring in one of a substantially horizontal orientation and a decliningorientation towards the axis of the ring; b) depositing the substance onthe upper surface; and c) imparting radial vibration to the ring at theresonant frequency of the ring, wherein a node is formed on the axis ofthe ring.